Variability and the concept of a probable climatic year in bioclimatology with reference to the Indian sub-continent

Abstract

The degree of stability of 31 stations of the sub-continent is analysed by consideration of four bioclimatic factors: number of rainy days, dry months, precipitation and its seasonal incidence (regime). Cases of exceptional deviations from the average values are pointed out. Taking coefficient of variation of the mean value above 32% as high variability for the first three factors, and less than 85% years in one of the four types of regimes (tropical, mediterranean, bixeric, irregular) as criterion of high variability in regime, the stations are graded according to the number of factors in which they show high variability. No correlation could be found between precipitation amount and variability though in case of rainy days and dry months an inverse relation has been noted with coefficient of variation, barring few exceptions. High annual variability might be contributed by all the seasons or the main agricultural season may be less affected. Stations are classified as presenting low, moderate, high variations in summer, winter, spring-autumn precipitation. In general, higher the variability, lesser the seasonal precipitation. A probable year is defined as one corresponding in precipitation, rainy days and dry months to the figures of mean value, plus or minus standard deviation. Percentage number of years corresponding to the pattern of probable year is noted. The percentage varies considerably for some stations when the regime of the most common type(s) is introduced as the fourth factor in the concept of probable year. No correlation exists between the percentage number of years agreeing with the probable year and the number of factors presenting high variability.